netdev - Re: [PATCH v2 bpf] bpf, test_run: fix &xdp_frame misplacement for LIVE

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Message-ID: <87ilg5vau3.fsf@toke.dk>
Date:   Mon, 13 Feb 2023 16:33:56 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 bpf] bpf, test_run: fix &xdp_frame misplacement for
 LIVE_FRAMES

Alexander Lobakin <alexandr.lobakin@...el.com> writes:

> &xdp_buff and &xdp_frame are bound in a way that
>
> xdp_buff->data_hard_start == xdp_frame
>
> It's always the case and e.g. xdp_convert_buff_to_frame() relies on
> this.
> IOW, the following:
>
> 	for (u32 i = 0; i < 0xdead; i++) {
> 		xdpf = xdp_convert_buff_to_frame(&xdp);
> 		xdp_convert_frame_to_buff(xdpf, &xdp);
> 	}
>
> shouldn't ever modify @xdpf's contents or the pointer itself.
> However, "live packet" code wrongly treats &xdp_frame as part of its
> context placed *before* the data_hard_start. With such flow,
> data_hard_start is sizeof(*xdpf) off to the right and no longer points
> to the XDP frame.
>
> Instead of replacing `sizeof(ctx)` with `offsetof(ctx, xdpf)` in several
> places and praying that there are no more miscalcs left somewhere in the
> code, unionize ::frm with ::data in a flex array, so that both starts
> pointing to the actual data_hard_start and the XDP frame actually starts
> being a part of it, i.e. a part of the headroom, not the context.
> A nice side effect is that the maximum frame size for this mode gets
> increased by 40 bytes, as xdp_buff::frame_sz includes everything from
> data_hard_start (-> includes xdpf already) to the end of XDP/skb shared
> info.
>
> Minor: align `&head->data` with how `head->frm` is assigned for
> consistency.
> Minor #2: rename 'frm' to 'frame' in &xdp_page_head while at it for
> clarity.
>
> (was found while testing XDP traffic generator on ice, which calls
>  xdp_convert_frame_to_buff() for each XDP frame)
>
> Fixes: b530e9e1063e ("bpf: Add "live packet" mode for XDP in BPF_PROG_RUN")
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>
> ---
> From v1[0]:
> - align `&head->data` with how `head->frm` is assigned for consistency
>   (Toke);
> - rename 'frm' to 'frame' in &xdp_page_head (Jakub);
> - no functional changes.
>
> [0]
> https://lore.kernel.org/bpf/20230209172827.874728-1-alexandr.lobakin@intel.com

Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>